The androgen receptor ("AR") is a ligand-activated transcriptional regulatory protein that mediates induction of male sexual development and function through its activity with endogenous androgens. Androgens are generally known as the male sex hormones. The androgenic hormones are steroids which are produced in the body by the testis and the cortex of the adrenal gland, or synthesized in the laboratory. Androgenic steroids play an important role in many physiologic processes, including the development and maintenance of male sexual characteristics such as muscle and bone mass, prostate growth, spermatogenesis, and the male hair pattern (Matsumoto, Endocrinol. Met. Clin. N. Am. 23:857-75 (1994)). The endogenous steroidal androgens include testosterone and dihydrotestosterone ("DHT"). Testosterone is the principal steroid secreted by the testes and is the primary circulating androgen found in the plasma of males. Testosterone is converted to DHT by the enzyme 5a-reductase in many peripheral tissues. DHT is thus thought to serve as the intracellular mediator for most androgen actions (Zhou, et al., Molec. Endocrinol. 9:208-18 (1995)). Other steroidal androgens include esters of testosterone, such as the cypionate, propionate, phenylpropionate, cyclopentylpropionate, isocarporate, enanthate, and decanoate esters, and other synthetic androgens such as 7-Methyl-Nortestosterone ("MENT") and its acetate ester (Sundaram et al., "7 Alpha-Methyl-Nortestosterone(MENT): The Optimal Androgen For Male Contraception," Ann. Med., 25:199-205 (1993) ("Sundaram")).
Because the AR is involved in male sexual development and function, the AR is a likely target for effecting male contraception or other forms of hormone replacement therapy.
Worldwide population growth and social awareness of family planning have stimulated a great deal of research in contraception. Contraception is a difficult subject under any circumstance. It is fraught with cultural and social stigma, religious implications, and, most certainly, significant health concerns. This situation is only exacerbated when the subject focuses on male contraception. Despite the availability of suitable contraceptive devices, historically, society has looked to women to be responsible for contraceptive decisions and their consequences. Although health concerns over sexually transmitted diseases has made men more aware of the need to develop safe and responsible sexual habits, women still often bear the brunt of contraceptive choice. Women have a number of choices from temporary mechanical devices such as sponges and diaphragms to temporary chemical devices such as spermicides. Women also have at their disposal more permanent options such as physical devices like IUDs and cervical caps as well as more permanent chemical treatments such as birth control pills and subcutaneous implants.
However, to date, the only options available for men include the use of condoms and a vasectomy. Condom use, however is not favored by many men because of the reduced sexual sensitivity, the interruption in sexual spontaneity, and the significant possibility of pregnancy caused by breakage or misuse. Vasectomies are also not favored. If more convenient methods of birth control were available to men, particularly long term methods which required no preparative activity immediately prior to a sexual act, such methods could significantly increase the likelihood that men would take more responsibility for contraception.
Despite prolonged research, spermatogenesis remains a poorly understood physiologic process. A number of strategies to interfere with normal hormonal control of spermatogenesis have been employed, including androgens, prostagens, and luteinizing hormone releasing hormone ("LHRH") analogs (Wu, "Male Contraception: Current Status and Future Prospects," Clin. Endocrin. 29:443:65 (1988) ("Wu 1988")). Administration of the male sex steroids (e.g., testosterone and its derivatives) has shown particular promise in this regard due to the combined gonadotropin-suppressing and androgen-substituting properties of these compounds (Steinberger et al., "Effect of Chronic Administration of Testosterone Enanthate on Sperm Production and Plasma Testosterone, Follicle Stimulating Hormone, and Luteinizing Hormone Levels: A Preliminary Evaluation of a Possible Male Contraceptive," Fertility and Sterility 28:1320-28 (1977)). Chronic administration of high doses of testosterone completely abolishes sperm production (azoospermia) or reduces it to a very low level (oligospermia). The degree of spermatogenic suppression necessary to produce infertility is not precisely known. However, a recent report by the World Health Organization showed that weekly intramuscular injections of testosterone enanthate result in azoospermia or severe oligospermia (i.e., less than 3 million sperm per ml) and infertility in 98% of men receiving therapy (World Health Organization Task Force on Methods for Regulation of Male Fertility, "Contraceptive Efficacy of Testosterone-Induced Azoospermia and Oligospermia in Normal Men," Fertility and Sterility 65:821-29 (1996)).
The main disadvantages of steroidal male contraceptives lie in their undesirable physicochemical and pharmacokinetic properties. Testosterone is readily absorbed after oral administration, but demonstrates little androgenic effect due to rapid hepatic metabolism prior to reaching the systemic circulation. Rapid absorption and metabolism also preclude the use of testosterone via intramuscular injection.
A variety of testosterone esters have been developed which are more slowly absorbed after intramuscular injection and, thus, result in greater androgenic effect. Testosterone enanthate is the most widely used of these esters. While testosterone enanthate has been valuable in terms of establishing the feasibility of hormonal agents for male contraception, it has several drawbacks, including the need for weekly injections and the presence of supraphysiologic peak levels of testosterone immediately following intramuscular injection (Wu, "Effects of Testosterone Enanthate in Normal Men: Experience From a Multicenter Contraceptive Efficacy Study," Fertility and Sterility 65:626-36 (1996)).
Steroidal ligands which bind the AR and act as androgens (e.g. testosterone enanthate) or as antiandrogens (e.g. cyproterone acetate) have been known for many years and are used clinically (Wu 1988). Although nonsteroidal antiandrogens are in clinical use for hormone-dependent prostate cancer, nonsteroidal androgens have not been reported. For this reason, research on male contraceptives has focused solely on steroidal compounds. However, data obtained during the development of nonsteroidal antiandrogens suggest that nonsteroidal androgens may be realized. In 1988, Tucker et al., ("Synthesis and Structure-Activity Relationships of 3-Substituted Derivatives of 2-Hydroxypropioanilides," J. Med. Chem. 31:954-59 (1988) ("Tucker 1988a")), using hydroxyflutamide as a lead compound, synthesized a large series of compounds with systematic structural modifications and tested their AR binding affinity and in vivo antiandrogenic activity. As a result of these studies, many of the structural elements required for nonsteroidal AR binding have been delineated (Tucker 1988a; Glen et al., "Structure-Activity Relationships Among Nonsteroidal Antiandrogens," Proceedings of the Third SCI-RSC Medicinal Chemistry Symposium, Royal Society of Chemistry: London, p. 345-61 (1986) ("Glen")). Glen showed that the AR binding affinity of a series of hydroxyflutamide analogs was dependent on the presence of an electron deficient aromatic ring separated from the tertiary carbinol by an amide link.
Electron-withdrawing groups presented at the 3- and 4- positions of the anilide ring further enhanced the AR binding affinity, particularly when the 4- substituent was cyano or nitro and the 3-substituent was chloro or trifluoromethyl. Physicochemical studies showed that the dominant conformation in these compounds was that in which the amide NH hydrogen bonds to the OH group, as opposed to that in which OH hydrogen bonds to the amidic carbonyl. This alignment between the NH and OH groups confers a planar geometry to the molecule and additional proton-donor ability to the OH group. It is also thought to play a crucial role in AR interaction of these compounds. This effect is also influenced by the electronic properties of other ring substituents (Glen). Tucker (Tucker 1988a; Tucker et al., "Resolution of Nonsteroidal Antiandrogen 4-Cyano-3-[(4-fluorophenyl) sulfonyl)]-2-hydroxy-2-methyl-3'-(trifluoromethyl)-propionanilide and Determination of the Absolute Configuration of the Active Enantiomer," J. Med. Chem. 31:885-87 (1988) ("Tucker 1988b")) synthesized and evaluated the in vivo activity of a series of arylthio derivatives, of which Casodex is an example. An infrared study on two representative sulfones showed that the dominant (&gt;95%) conformation in nonpolar solvents is a structure in which the OH group is bound intramolecularly to a sulfonyl group oxygen atom (Morris, et al., "Non-Steroidal Antiandrogens: Design of Novel Compounds Based on an Infrared Study of Dominant Conformation and Hydrogen Bonding Properties of a Series of Anilide Antiandrogens," J. Med. Chem. 34:447-55 (1991)). This suggests that the OH group is not free to participate in AR interactions and that a different binding mechanism may be operative for these compounds. However, an NMR study by the same group indicated that the hydrogen bond to the sulfonyl group is disrupted in proton acceptor solvents, such as the aqueous environment of the AR, and enables the OH group to interact as originally proposed (Tucker 1988a). Teutsch et al., "Non-Steroidal Antiandrogens: Synthesis and Biological Profile of High Affinity Ligands for the Androgen Receptor," J. Steroid Biochem. Molec. Biol. 48:111-19 (1994) recently reported a series of N-substituted arylthiohydantoin derivatives with AR binding affinities over 100-fold greater than those reported for flutamide, nilutamide, and Casodex. The planar geometry of these compounds corroborates the significance of this conformation with regard to AR interaction with nonsteroidal ligands. Casodex, recently given the generic name bicalutamide, was developed as a result of these studies, and has several advantages over other antiandrogen therapies (Tucker 1988a; Tucker 1988b). Casodex has about 2 to 4 times the AR binding affinity of flutamide and nilutamide, greater in vivo potency, improved tolerance, a long biological half-life (6 days in man) compatible with once daily dosing, and is well absorbed after oral administration (Neri, et al., "Effects of a Novel Non-Steroidal Antiandrogen on Canine Prostatic Hyperplasia," Invest. Urol. 10:123-130 (1972); Furr, A Novel Non-Steroidal Peripherally Selective Antiandrogen. In: Management of Advanced Cancer of Prostate and Bladder," New York, A. R. Liss, pp. 13-26 (1988); Kennealey, et al., "Use of the Non-Steroidal Antiandrogen Casodex in Advanced Prostatic Carcinoma," Urol. Clin. N. Am. 18(1):99-110 (1991)).
In addition to antiandrogens, these studies uncovered a number of structural analogs demonstrating partial agonist activity. Tucker (1988a) showed that substitution with a trifluoromethyl 3-substituent apparently resulted in partial agonist activity, and that partial agonist activity was enhanced when the 4-substituent was nitro, as opposed to cyano. However, it is important to note that the objective of the Tucker studies was to identify nonsteroidal antiandrogens for treatment of prostate cancer. Although several partial agonists were identified, the structural properties required for androgenic activity were not further investigated. In addition, Tucker measured in vivo functional activity. As such, a compound which produces potent androgenic effects upon interaction with the AR but is rapidly metabolized would demonstrate poor in vivo agonism and, therefore, interfere with the identification of valid structure activity relationships. Thus, the animal model used by Tucker was not designed for and was incapable of identifying compounds with AR agonist activity. Molecular cloning and characterization of the cDNA encoding the AR (Lubahn, et al., "The Human Androgen Receptor: Complementary Deoxyribonucleic Acid Cloning, Sequence Analysis and Gene Expression in Prostate," Molec. Endocr. 2:1265-75 (1988); Chang, et al., "Structural Analysis of Complementary DNA and Amino Acid Sequences of Human and Rat Androgen Receptors," PNAS 85:7211-15 (1988); Trapman, et al., "Cloning, Structure and Expression of a cDNA Encoding the Human Androgen Receptor," Biochem. Biophys. Res. Commun. 153:241-48 (1988)) now permit direct measurement of the functional activity of AR ligands in the absence of potentially confounding pharmacokinetic factors. Also, Tucker utilized racemic mixtures of the compounds of interest for their in vitro and in vivo experiments. This is of particular significance given a recent report regarding the stereoselective pharmacokinetics of Casodex (McKillop, et al., "Metabolism and Enantioselective Pharmacokinetics of Casodex in Man," Xenobiotica 23:1241-1253 (1993) ("McKillop")). McKillop showed that the elimination half-life of R-Casodex (4.2 days) was significantly longer than that of the S-enantiomer (t.sub.1/2, 19 hrs) in healthy male volunteers receiving a single oral dose of the racemic mixture. Further, area under the plasma concentration-time profile (AUC) for the R-enantiomer was 130-fold greater than that of S-Casodex, indicating a significant difference in physiologic exposure to the two isomers. Differences in functional activity reported by Tucker 1988a and 1988b did not account for potential stereoselective differences in drug metabolism.
In summary, many of the structural elements and geometry required for nonsteroidal ligand binding to the AR have been determined. However, a significant knowledge gap exists with regard to nonsteroidal agonists, due to the fact that previous studies focused on elucidation of structure-activity relationships for receptor antagonists and used test systems providing potentially misleading results. Studies utilizing stereochemically pure compounds and more specific measures of functional activity were necessary to elucidate the structure-activity relationship for nonsteroidal agonism.
The present invention is directed toward overcoming these above-noted deficiencies.